The present invention relates to a new erythromycin A 9-oxime compound which can effectively be used as an intermediate for the preparation of 6-O-alkyl erythromycin A such as 6-O-methyl erythromycin A (hereinafter referred to as xe2x80x9cclarithromycinxe2x80x9d) or its oxime, a process for preparing the same and a process for preparing 6-O-alkyl erythromycin A or its oxime using the same.
6-O-alkyl erythromycin A is a semi-synthetic macrolide antibacterial agent showing an excellent antibacterial activity against a number of bacteria which can cause diseases in human or mammals, for example, gram-positive bacteria, a part of gram-negative bacteria, anaerobic bacteria, Mycoplasma, and Chlamidia, etc., and also can be used as a raw material for the synthesis of other antibiotic in this field, and it therefore is a pharmaceutically important material.
6-O-alkyl erythromycin A compound including 6-O-methyl erythromycin A can be prepared by selectively alkylating the 6-position of suitably protected erythromycin A 9-oxime, eliminating the protecting groups and then carrying out deoximation. Since erythromycin A 9-oxime has a number of reactive hydroxy groups including the hydroxy group of oxime and a dimethylamino group at the 3xe2x80x2 position which can also participate in alkylation, it is very important to carry out alkylation of the hydroxy group at the 6 position after protecting these groups with suitable substituents.
Prior art processes for preparing 6-O-alkyl erythromycin A, especially 6-O-methyl erythromycin A from erythromycin A 9-oxime are disclosed in EP patent Nos. 0,158,467, 0,195,960, and 0,272,110 and U.S. Pat. No. 5,719,272 and WO 97/36913.
Separately from the above processes, processes for preparing it from erythromycin A derivatives are disclosed in EP Patent Nos. 0,147,062 and 0,177,696, but these are not practical.
Known processes for preparing 6-O-alkyl erythromycin A compound from erythromycin A 9-oxime can be summarized as follows:
(1) The first method comprises protecting oxime hydroxy group of erythromycin A 9-oxime with an alkyl group, etc. and protecting the hydroxy group at the 2xe2x80x2 position and dimethylamino group at the 3xe2x80x2 position with benzyloxycarbonyl groups, thereafter carrying out alkylation such as methylation of the hydroxy group at the 6 position.
However, this process has some problems in industrial use since benzyloxycarbonyl chloride used as the protecting group is expensive and must be used in an excess amount. Furthermore, it has a drawback that it requires an additional step for regenerating the dimethylamino group at the 3xe2x80x2 position after carrying out alkylation of the hydroxy group at the 6 position.
(2) The second process comprises protection of the oxime hydroxy group and the hydroxy group at the 2xe2x80x2 position of erythromycin A 9-oxime with benzyl groups, together with quarternization of the dimethylamino group at the 3xe2x80x2 position with the same protecting group, and then alkylation of the hydroxy group at the 6 position.
However, this process has a drawback that, after carrying out alkylation of the hydroxy group at the 6 position, the protecting group at the 2xe2x80x2 position is not readily removed in the step of deprotection of the benzyl group by hydrogenation due to catalytic poison phenomenon.
(3) The third process comprises protection of the oxime hydroxy group of erythromycin A 9-oxime with a ketal compound such as cyclohexanone diisopropyl ketal, protection of the hydroxy groups at the 2xe2x80x2 and 4xe2x80x2 positions with silyl groups, and then carrying out alkylation of the hydroxy group at the 6 position. This process has advantages that it does not require protection of the dimethylamino group at the 3xe2x80x2 position, and it is possible to eliminate the protecting groups at one time after carrying out alkylation. However, it has a drawback that ketal compounds which are used in protecting the hydroxy group of oxime should be separately synthesized.
(4) The fourth method comprises carrying out protection of all of the oxime hydroxy group and the hydroxy groups at the 2xe2x80x2 and 4xe2x80x2 positions of erythromycin A 9-oxime with silyl groups, and then carrying out alkylation of the hydroxy group at the 6 position. However, this process has a number of drawbacks that since the silyl group is quite unstable as the protecting group for oxime, the hydroxy group of oxime is also alkylated during the alkylation reaction for the hydroxy group at the 6 position.
Therefore, the present inventors have extensively conducted a research in order to solve the above mentioned drawbacks involved in the prior art techniques and to prepare 6-O-alkyl erythromycin A in a high yield. As a result, we found that 6-O-alkyl erythromycin A of the general formula (II) and its oxime can conveniently be prepared in a high yield with low cost by a process wherein two molecules of the hydroxy groups of erythromycin A 9-oxime is first protected with one molecule of protecting group using the protecting groups which can react with oxime at two positions to give a erythromycin A 9-oxime compound in symmetric structure; and then up to four hydroxy groups of erythromycin A 9-oxime compound corresponding to the 2xe2x80x2 and 4xe2x80x3 positions of erythromycin A are protected with silyl protecting groups; and the hydroxy group at the 6 position is selectively alkylated; and finally deprotection and deoximation are carried out. 
wherein,
R represents hydrogen or a C1-3 alkyl group; and
X represents O or Nxe2x80x94OH.
The present invention provides a process for preparing 6-O-alkyl erythromycin A 9-oxime and 6-O-alkyl erythromycin A represented by the following formula (II) which comprises the steps (a) to (e):
(a) reacting erythromycin A 9-oxime represented by the following formula (III) with a bifunctional linker represented by the following formula (IV) to give an erythromycin A 9-oxime compound represented by the following formula (V);
(b) silylating the erythromycin A 9-oxime compound of formula (V) obtained in the step (a) to give an erythromycin A 9-oxime compound represented by the following formula (VI);
(c) alkylating the compound of formula (VI) obtained in the step (b) to give an erythromycin A 9-oxime compound represented by the following formula (VII);
(d) desilylating the erythromycin A 9-oximie compound of formula (VII) obtained in step (c) to give an erythromycin A 9-oxime compound represented by the following formula (VIII); and
(e) removing the oxime protecting group of the compound of formula (VIII) obtained in the step (d) or consecutively carrying out removal of the oxime protecting group and deoximation reaction: 
xe2x80x83wherein,
R represents a C1-3 alkyl group;
R1 and R2 each independently represents a silyl group represented by xe2x80x94SiR4R5R6 which R4, R5, and R6 each independently represents a C1-6 alkyl group, a C1-3 alkyl group substituted with a phenyl group, a phenyl group, a C5-7 cycloalkyl group, or a C2-5 alkenyl group; or R2 represents hydrogen;
R3 represents hydrogen, a 1-alkenyl group, or an aryl group;
Q represents a cis- or trans-alkenylene group represented by xe2x80x94(CHxe2x95x90CH)mxe2x80x94, a xcex1,xcfx89-dienylene group represented by xe2x80x94[CHxe2x95x90CHxe2x80x94(CH2)mxe2x80x94CHxe2x95x90CH]xe2x80x94, a phenyl group consisting of ortho-, meta- or para-, etc., an arylene group such as naphthalenyl, anthracenyl or pyridinyl group, etc. or an alkylene group represented by xe2x80x94(CH2)nxe2x80x94 when R3 represents a 1-alkenyl group or an aryl group, m is an integer of 1 to 3, and n is an integer of 1 to 10, wherein one or more hydrogens in the above alkylene group, alkenylene group, dienylene group or arylene group may be substituted with a suitable alkyl substituent;
X represents O or Nxe2x80x94OH; and
Y represents a leaving group such as chloride, bromide, iodide, mesylate, tosylate and triflate.
The present invention also provides an erythromycin A 9-oxime compound represented by the following formula (I) which can be used as an intermediate for the preparation of 6-O-alkyl erythromycin A 9-oxime and 6-O-alkyl erythromycin A represented by the formula (II). 
wherein,
R represents hydrogen, or a C1-6 alkyl group,
R1 and R2 each independently represents hydrogen or a silyl group represented by xe2x80x94SiR4R5R6 in which R4, R5, and R6 each independently represents a C1-6 alkyl group, a C1-3 alkyl group substituted with a phenyl group, a phenyl group, a C5-7 cycloalkyl group, or a C2-5 alkenyl group; or
R3 represents hydrogen, a 1-alkenyl group, or an aryl group;
Q represents a cis- or trans-alkenylene group represented by xe2x80x94(CHxe2x95x90CH)mxe2x80x94, a xcex1,xcfx89-dienylene group represented by xe2x80x94[CHxe2x95x90CHxe2x80x94(CH2)mxe2x80x94CHxe2x95x90CH]xe2x80x94, a phenyl group consisting of ortho-, meta- or para-, etc., an arylene group such as naphthalenyl, anthracenyl or pyridinyl group, etc. or an alkylene group represented by xe2x80x94(CH2)nxe2x80x94 when R3 represents a 1-alkenyl group or an aryl group, m is an integer of 1 to 3, and n is an integer of 1 to 10, wherein one or more hydrogens in the above alkylene group, alkenylene group, dienylene group or arylene group may be substituted with a suitable alkyl substituent.
Hereinafter, the invention will be illustrated in more detail.
Erythromycin A 9-oxime compound represented by the formula (I) according to the present invention is an intermediate in which two molecules of erythromycin A 9-oxime is connected to a specific group of the bifunctional linker serving as a protecting group of oxime. Using this compound, the desired 6-O-alkyl erythromycin A and its oxime of formula (II) can be conveniently prepared in a high yield.
In the erythromycin A 9-oxime compound of formula (I) according to the present invention, preferred are those in which Q represents a 1,2-diphenylethyl group, a phenyl group consisting of ortho-, meta- or para-, a 1,4-naphthalenyl group, a cis- or trans-ethylene group or a butadienyl group; R represents hydrogen or a methyl group; R1 represents a trimethyl silyl group, R2 represents hydrogen or a trimethylsilyl group; and R3 represents hydrogen.
In the erythromycin A 9-oxime compound of formula (I) according to the present invention, more preferred are those in which Q represents a para-phenyl group; R represents a methyl group; R1 represents a trimethylsilyl group, R2 represents hydrogen or trimethylsilyl group; and R3 represents hydrogen.
The term used herein, xe2x80x9calkylxe2x80x9d includes an alkyl group having carbon number of 1 to 3.
According to the present invention, the process for preparing 6-O-alkyl erythromycin A and its 9-oxime represented by the formula (II) via an erythromycin A 9-oxime compound represented by the formula (I) from the erythromycin A 9-oxime of formula (III) can be accomplished by separately conducting several reaction steps or consecutively or concurrently conducting two or more necessary reaction steps without isolating the desired products in each step. Therefore, the present invention encompasses the two methodologies which will be explained in more detail.
Step (a)
In this step, erythromycin A 9-oxime of formula (III) is reacted with bifunctional linker of formula (IV) in a solvent under the presence of a base to give an erythromycin A 9-oxime compound represented by the formula (V). 
wherein,
R3 represents hydrogen, a 1-alkenyl group or an aryl group;
Q represents a cis- or trans-alkenylene group represented by xe2x80x94(CHxe2x95x90CH)mxe2x80x94, a xcex1,xcfx89-dienylene group represented by xe2x80x94[CHxe2x95x90CHxe2x80x94(CH2)mxe2x80x94CHxe2x95x90CH]xe2x80x94, a phenyl group consisting of ortho-, meta- or para-, etc., an arylene group such as naphthalenyl, anthracenyl or a pyridinyl group, etc., or an alkylene group represented by xe2x80x94(CH2)nxe2x80x94 when R3 represents a 1-alkenyl group or an aryl group, m is an integer of 1 to 3, n is an integer of 1 to 10, wherein one or more hydrogens in the above alkylene group, alkenylene group, dienylene group or arylene group may be substituted with a suitable alkyl substituent; and
Y represents a leaving group such as chloride, bromide, iodide, mesylate, tosylate and triflate.
The bifunctional linker represented by the formula (IV) which can be used in the step (a) includes, for example, an aryl substituted alkyl group such as 1,2-dibromo-1,2-diphenylethane, an alkenyl group such as 1,4-dichloro-2-butene, and an aryl group such as xcex1,xcex1xe2x80x2-dichloro-o-xylene, xcex1,xcex1xe2x80x2-dichloro-m-xylene, xcex1,xcex1xe2x80x2-dichloro-p-xylene, 2,4-bis(chloromethyl)-1,3,5-trimethylbenzene, 2,6-bis(chloromethyl)pyridine etc. It is preferable to employ this bifunctional linker in an amount of about 0.5 mole equivalent for erythromycin A 9-oxime.
As the solvent used in the step (a), preferred are aprotic polar solvent or the mixture thereof such as acetone, acetonitrile, 1,2-dimethoxyethane, tetrahydrofuran, 1,4-dioxane, N,N-dimethylformamide, N,N-dimethylacetamide, dimethylsulfoxide, sulfolane, N,N,Nxe2x80x2,Nxe2x80x2,Nxe2x80x3,Nxe2x80x3-hexamethyl-phosphoramide, etc. Also, as the base, for example, sodium hydride, potassium hydride, sodium hydroxide, potassium hydroxide, potassium t-butoxide, sodium carbonate, potassium carbonate, etc. may be preferably used. It is sufficient to use the base in an amount of about 1 to 1.5 mole equivalents for erythromycin A 9-oxime. The reaction is carried out at about xe2x88x9215xc2x0 C. to 50xc2x0 C., preferably 0xc2x0 C. to room temperature under stirring. After completion of the reaction, reactants are dispersed into water and then the resulting solid is filtered and dried to give the desired product (V) conveniently.
The erythromycin A 9-oxime of formula (III) which is reacted with the bifunctional linker of formula (IV) according to the present invention may be present as two xe2x80x98synxe2x80x99 form and xe2x80x98antixe2x80x99 form isomers. The isomer in the present invention may be any of them or a mixture thereof, with anti-form isomer being preferred.
Step (b)
The erythromycin A 9-oxime compound represented by the above formula (V) is reacted with various silylating agents to prepare an erythromycin A 9-oxime compound represented by the formula (VI) in which two hydroxy groups at the 2xe2x80x2 position and two hydroxy groups at the 4xe2x80x3 position of the above formula (V) corresponding to the 2xe2x80x2 and 4xe2x80x2 positions of erythromycin A are protected with silyl groups. 
wherein,
Q and R3 are as defined above,
R1 and R2 each independently represents a silyl group represented by xe2x80x94SiR4R5R6 in which R4, R5, and R6 each independently represents a C1-6 alkyl group, a C1-3 alkyl group substituted with a phenyl group, a phenyl group, a C5-7 cycloalkyl group, or a C2-5 alkenyl group; or R2 represents hydrogen.
The examples of the silylating agent for the compound of formula (V) include chlorosilanes such as trimethylchlorosilane, triethylchlorosilane, and tert-butyldimethylchlorosilane; silylamines such as 1,1,1,3,3,3-hexamethyldisilazane, trimethylsilylimidazole, and N,N-dimethylaminotrimethylsilane; and silylamides such as bis(trimethylsilyl)acetamide, trimethylsilyl diphenylurea, and bis(trimethylsilyl)urea, etc. When chlorosilanes are used, it is desirable to carry out the reaction under the presence of a base, examples of which include inorganic base such as sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium bicarbonate, potassium bicarbonate and organic base such as trimethylamine, triethylamine, tri-n-butylamine, diisopropylethylamine, pyridine, N,N-dimethylamine, imidazole, 1,8-diazabicyclo[5,4,0]undec-7-ene, etc. When silylamines are used, it is preferable to use together with a weak acid salt such as ammonium chloride, pyridine hydrochloride, pyridine p-toluenesulfonate, etc. The amount of silylating agent to be used is about 2 to 20 mole equivalents with respect to the compound of formula (V). The reaction is coined out at 0xc2x0 C. to reflux temperature of the solvent, preferably at about 10xc2x0 C. to 50xc2x0 C. in a solvent under stirring. As the solvent, dichloromethane, 1,2-dichloroethane, chloroform, acetone, acetonitrile, tetrahydrofuran, dioxane, 1,2-dimethoxymethane, N,N-dimethylformamide, N,N-dimethylacetamide, dimethyl sulfoxide, etc. can be used. Depending upon the kind and amount of the silylating agent to be used in the step (b), it is possible to silylate only two hydroxy groups of the compound of formula (V) which corresponds to the 2xe2x80x2 position of erythromycin A or to silylate the two hydroxy groups of the compound of formula (V) which correspond to the 2xe2x80x2 position and any of the two hydroxy groups of the compound of formula (V) which correspond to the 4xe2x80x3 position of erythromycin A.
In preparing the compound of formula (VI) according to the present invention, it is possible to consecutively carry out the reaction steps (a) and (b) from the compound of formula (II) in the same vessel without isolating the compound of formula (V).
Step (c)
The erythromycin A 9-oxime compound represented by the formula (VI) is reacted with various alkylating agents to prepare an erythromycin A 9-oxime compound represented by the following formula (VIl) in which two hydroxy groups of the compound of formula (VI) corresponding to the 6 position of erythromycin A are alkylated. 
wherein.
Q R1, R2 and R3 are as defined above, and R represents a C1-3 alkyl group.
The reaction of the compound of formula (VI) and the alkylating agent is carried out at xe2x88x9215xc2x0 C. to 40xc2x0 C., preferably about 0xc2x0 C. to room temperature under the presence of a base in a solvent under stirring. As the alkylating agent, methyl bromide, methyl iodide, ethyl bromide, ethyl bromide, ethyl iodide, propyl bromide, propyl iodide, allyl bromide, dimethyl sulfate, p-toluenesulfonylmethane, methanesulfonylmethane, etc. can be used. The amount of alkylating agent to be used is about 2 to 4 mole equivalents with respect to the compound of formula (VI). The solvent which can be used in this reaction includes aprotic polar solvents such as N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrolidone, dimethyl sulfoxide, N,N,Nxe2x80x2,Nxe2x80x2,Nxe2x80x3,Nxe2x80x3-hexamethylphosphoramide, etc. and the mixture thereof or a mixed solvent consisting of one solvent selected from the above aprotic solvent and another solvent selected from the group consisting of tetrahydrofuran, 1,2-dimethoxyethane, dioxane, acetonitrile, etc. As the base, sodium hydride, potassium hydride, sodium hydroxide, potassium hydroxide, potassium t-butoxide, etc. may be used. The base is used in an amount of about 2 to 4 mole equivalents with respect to the compound of formula (VI).
It is well known that when alkylating two hydroxy groups of formula (VI) corresponding to the 6 position of erythromycin A according to the present invention, the condition required for preventing 3xe2x80x2-dimethylamino group from becoming a quarternary salt is generally to protect the 2xe2x80x2-hydroxy group with a silyl group and the protection of 4xe2x80x3-hydroxy group with a silyl group is not an essential condition. Upon alkylation reaction of the two hydroxy groups at the 6 position of formula (VI) according to the present invention, N-methylation hardly occurs at the two 3xe2x80x2-dimethylamino groups of formula (VI). Therefore, the protection of the two 3xe2x80x2-dimethylamino groups is not required.
Step (d)
The erythromycin A 9-oxime compound represented by the formula (VII) may be converted into an erythromycin A 9-oxime compound represented by the formula (VIII) by removing silyl groups which are up to 4 hydroxy protecting groups corresponding to the 2xe2x80x2 and 4xe2x80x3 positions of erythromycin A. 
wherein,
Q and R3 are as defined above, and R is a C1-3 alkyl group.
Generally, the method for removing silyl groups serving as the hydroxy protecting groups of hydroxy group is well known. For example, this protecting group can be readily removed by treating with an organic acid such as formic acid or an inorganic acid such as hydrochoric acid in an organic solvent such as alcohols or in a mixed solvent of water and the organic solvent or by tetrabutylammonium fluoride, etc. in an organic solvent such as tetrahydrofuran.
Step (e)
6-O-alkyl erythromycin A 9-oxime represented by the formula (II) in which X represents Nxe2x80x94OH can be prepared from the compound of formula (VIII) obtained in step (d) by removing oxime protecting groups of two molecules of 6-O-alkyl erythromycin A 9-oximes.
Removal of the protecting groups which protect two oximes in the compound of formula (VIII) can easily be carried out by reductive hydrogenation reaction of homogeneous or heterogeneous system. For example, this reaction can be carried out in alcohols such as methanol, ethanol, etc. or in an aqueous solution of alcohols under the presence of palladium or palladium-carbon catalyst under hydrogen atmosphere with stirring. At this time, it may be possible to add formic acid, acetic acid, etc. in order to prevent catalyst poison due to the nitrogen atom of 3xe2x80x2-dimethylamino group and to easily carry out the reaction.
The reaction may also be carried out in methanol, ethanol, and N,N-dimethylformamide using ammonium formate, sodium formate or the mixture of the formate with formic acid which serves as the hydrogen source under the presence of palladium or palladium-carbon catalyst at room temperature to 100xc2x0 C. with stirring.
This reaction can also be carried out using a suitable catalyst in transition elements of VIIIB group or a catalyst consisting of the elements and a suitable ligand. As the example of transition elements, ruthenium, rhodium, palladium and platinum may be listed. The catalyst can be used as a salt or a complex. As the ligand, triphenylphosphine, tri-n-butylphosphine, triethylphosphite, 1,2-ethylene(diphenyl)-phosphine, etc. may be used. Typically, a combined catalyst of palladium and triphenylphosphine is used. This reaction may be carried out in formic acid or the salt thereof. As the example of the salt, ammonium salts such as ammonium formate, trimethylammonium formate or alkali metal salts such as sodium formate, potassium formate, etc. may be listed.
In addition to the above process in which the oxime protecting functional groups are removed from the compound of formula (VIII) to prepare 6-O-alkyl erythromycin A 9-oxime represented by the formula (II) in which X represents Nxe2x80x94OH, the compound of formula (II) may be prepared starting from the compound of formula (VII) by concurrently carrying out the steps of (d) and (e). The same hydrophilic solvent as used in the steps (d) and (e) may be used in these reactions. In these reactions, the order of the removal of substituents R1 and R2 and the removal of the oxime protecting functional groups may be reversed.
The thus obtained 6-O-alkyl erythromycin A 9-oxime of formula (II) in which X represents Nxe2x80x94OH can be deoximized in accordance with the known process, for example, as disclosed in EP Nos. 0,158,467 and 0,195,960, using inorganic oxidized sulfur compounds such as sodium hydrogen sulfite, sodium pyrothiosulfate, sodium thiosulfate, sodium hydrosulfite, sodium metabisulfite, sodium dithionate, potassium hydrogen sulfite, potassium thiosulfate, potassium metabisulfite, trichlorotitanium-ammonium acetate, sodium nitrite-hydrochloric acid, etc. to prepare 6-O-alkyl erythromycin A of formula (II) in which X represents O.
According to the present invention, however, in order to shorten the reaction steps and to efficiently prepare 6-O-alkyl erythromycin A of formula (II) in which X represents O, it is possible to concurrently or consecutively carry out the desilylation reaction of the step (d) from the compound of formula (VIl), the removal of oxime protecting groups in step (e) and the deoximation reaction, or if is also possible to concurrently or consecutively carry out deprotection reaction of oxime protecting groups in step (e) from the compound of formula (VIII) and deoximation reaction. The same hydrophilic solvent as used in the steps (d) and (e) may be used in these reactions. For example, the compound of formula (VII) is reacted with formic acid, ammonium formate, palladium-carbon and sodium hydrogen sulfite in methanol, ethanol or an aqueous solution thereof at 50xc2x0 C. to the refluxing temperature of solvent to prepare 6-O-alkyl erythromycin A of formula (II) including the commercial antibiotic, 6-O-methyl erythromycin A. The amount of formic acid to be used is about 10 to 100 mole equivalents, preferably, about 20 to 50 mole equivalents and the amount of ammonium formate is about 2 to 20 mole equivalents, preferably about 4 to 10 mole equivalents with respect to the compound of formula (VII). Palladium is used in an amount of about 0.01 to 0.5 mole equivalents in the standard of 1% to 20% regardless of content of moisture. The amount of sodium hydrogen sulfite is about 2 to 30 mole equivalents, preferably about to 8 to 20 mole equivalents.
The compounds represented by the formulas (V), (VI), (VII) and (VIII) which provided in order to illustrate each reaction step according to the present invention are encompassed by the compound of formula (I) in their scope. Therefore, it is apparent that the compounds of formulas (V), (VI), (VII) and (VIII) are encompassed by the compound of formula (I).